1. Field of the Invention
The present invention relates to methods of making fluoride crystals and fluoride crystal lenses. In particular, the present invention relates to a method of making a fluoride crystal which is used in optical articles such as lenses which are transparent to excimer laser beams and the like.
2. Description of the Related Art
Excimer lasers have attracted attention as the only high-output laser which can oscillate in an ultraviolet region, and have been applied in electronic, chemical and energy industrial fields. In these industrial fields, the excimer laser is used for processing metal, resin, glass, ceramic and semiconductor articles as well as for chemical reactions.
The equipment for generating an excimer laser beam is known as an excimer laser oscillator. A laser gas such as Ar, Kr, Xe, F2, or Cl2 is filled into a chamber and is excited by electron beam irradiation or electrical discharge. The excited atoms bond to atoms in a ground state to form molecules which can be present in an excited state. Such molecules are called excimers. The excimers are unstable and immediately return to the ground state, simultaneously emitting ultraviolet light. This phenomenon is referred to as bond-free transition. An excimer laser oscillator amplifies ultraviolet light formed by the bond-free transition with an optical resonator comprising a pair of mirrors and outputs it as a laser light beam.
Among excimer laser beams, the KrF laser beam and the ArF laser beam operate in a vacuum ultraviolet region and have wavelengths of 248 nm and 193 nm, respectively. Optical articles must have high transparencies in such a region. Fluorides such as calcium fluoride are suitable for such optical articles. Known fluorides include calcium fluoride, magnesium fluoride, barium fluoride, neodymium fluoride, lithium fluoride and lanthanum fluoride.
Conventional methods of making fluoride crystals will be illustrated using calcium fluoride (CaF2), referred to as fluorite, as a typical example. Methods of making fluoride crystals are disclosed in, for example, Japanese Unexamined Patent Publication Nos. 4-349,198 and 4-349,199. When high purity powdery raw material prepared by a synthetic process is melted using these methods, the volume of the raw material vastly decreases during melting in a crystal growing furnace due to the low bulk density of the powder. Thus, a high purity crushed solid material like cullet is used in the crucible to avoid such volume reduction.
FIG. 8 is a flow chart illustrating a method of making a fluoride crystal which the present inventors used before they discovered the present invention. Raw powder is prepared in step S1, melted and then cooled in a container as shown in step S2. The solid bulk is crushed with a stainless steel crusher in step S3. The crushed solid is placed and melted in a crucible for crystal growth, and gradually cooled to grow a fluoride crystal in step S4. Step S2 is to reduce the bulk density change which occurs during melting in step S4. The resulting fluoride crystal is shaped into a lens and the like, and is used as an optical article.
Although the fluoride crystal obtained by the method set forth above exhibits more satisfactory characteristics in general visible light optical articles than those obtained by other prior art methods, its optical characteristics deteriorate during repeated irradiation cycles of high output light having a short wavelength, such as an excimer laser beam.
The present inventors have found that the deterioration is induced by the structure of and the impurities in the fluoride crystal resulting from the method of making the fluoride crystal. It was found that large amounts of impurities, e.g. water, iron, nickel and chromium, are included in the fluoride raw material of step S3 set forth above, and these impurities cause characteristic deterioration during consecutive irradiation of high-output short-wavelength light over long time periods.
Further, in the above-mentioned method, since the bulk density of the fluoride decreases in the process from step S3 to step S4, only a relatively small size fluoride crystal can be obtained unless a large crystal growing furnace is used. Thus, this method inefficiently produces fluoride crystal and has high production costs. If large crystal production is sought using this method, a larger furnace must be provided, resulting in an increase in start-up costs. Moreover, the melting step S2 and the crushing step S3 increase the production period and decrease throughput.